Method for operating a lean burn engine with an aftertreatment system including nonthermal plasma discharge device

ABSTRACT

In an exhaust aftertreatment system, which includes a nonthermal plasma discharge device followed by a NOx storage device followed by a precious metal catalyst, a method is disclosed for operating the engine at a lean air-fuel ratio when the NOx storage device is not full and for operating the engine rich when the NOx storage device is substantially full. Electrical energy supplied to the nonthermal plasma discharge device during lean operation is discontinued during rich operation of the engine.

FIELD OF INVENTION

The present invention relates to an aftertreatment system for treatingexhaust gases from a lean burn engine and, more particularly, to anaftertreatment system comprising a nonthermal plasma discharge device.

BACKGROUND OF THE INVENTION

It is well known by those skilled in the art that internal combustionengines burning a lean mixture of fuel and air consume significantlyless fuel than when operating at a stoichiometric mixture of air andfuel. Presently, there are very few lean burn gasoline engines inproduction due to difficulties in meeting emission requirements. Thedifficulty is that conventional precious metal catalyst oxidizes CO andhydrocarbons and reduces NOx at high efficiency when the air-fuelmixture is very close to stoichiometric; but, NOx conversion efficiencydrops off substantially when the exhaust gases are lean.

It is known in the art to use a lean NOx trap (LNT) aftertreatmentsystem for processing the products of lean combustion. During leancombustion, NOx is trapped in the LNT. When the LNT is full, the engineis operated rich for a short period of time. The rich exhaust gasescause the absorbed NOx to desorb from catalyst surfaces. Furthermore,the rich exhaust gases contain CO and unburned hydrocarbons that reduceNOx to N2. Although commonly called a lean NOx trap, the LNT actuallystores only NO2 to a high degree. Because NOx coming from the engine ispredominantly comprised of NO and very little NO2, an oxidation catalystis provided upstream to cause NO to oxidize to NO2.

The inventors of the present invention have recognized a difficulty inrelying on an oxidation catalyst to perform the oxidation of NO to NO2.Specifically, the oxidation catalyst is only partially active below atemperature of about 200° C. Thus, during warm up or at very low powerconditions, the reaction from NO to NO2 is marginal. Consequently, NOproceeds through the LNT and out the vehicle tailpipe unprocessed.

A known problem with lean NOx traps is their susceptibility to SOxcontamination. Most hydrocarbon fuels contain some sulfur. The sulfuroxidizes mostly to SO2 during the combustion process in the combustionchamber. If an oxidation catalyst is placed upstream of the LNT, the SO2is further oxidized to SO3. SO2 can pass through the exhaust system withno harmful effect. However, SO3, in the presence of water vapor in theexhaust, forms particulates containing sulfuric acid. These becomeabsorbed in the LNT and reduce its conversion efficiency. To overcomesulfur degradation of LNT performance, it is known to periodicallyregenerate the trap, commonly called deSOx. The SOx can be desorbed andmade to exit the LNT when its temperature is raised to a hightemperature, in the range of 700–800° C., for a period of time,typically greater than a minute. The inventors of the present inventionhave recognized several problems with sulfur contamination: first, theLNT operates at less than its optimal efficiency for much of the timedue to the sulfur contamination and secondly, the deSOx operation iscumbersome, penalizes fuel economy, and the deSOx temperature is nearthe temperature at which permanent damage to the LNT occurs makingcontrol of deSOx regeneration a challenge. Furthermore, deSOxregeneration is not completely reversible. The propensity of anoxidation catalyst to oxidize SO2 to SO3 is harmful to the LNT. SomeLNTs contain precious metals, such as platinum, in their formulation. Insuch LNTs, the oxidation of SO2 to SO3 happens regardless of whetherthere is an oxidation catalyst upstream or not.

The inventors of the present invention have further recognized that itis desirable to provide any aftertreatment system with an onboarddiagnostic procedure to detect system deficiencies.

SUMMARY OF THE INVENTION

To overcome disadvantages in the prior systems, the inventors of thepresent invention have recognized that a nonthermal plasma dischargedevice can be used to convert NO to NO2 in place of an oxidationcatalyst.

Disclosed herein is a method of operating an internal combustion enginecoupled to an exhaust aftertreatment system, the aftertreatment systemincluding a nonthermal plasma discharge device followed by a NOx storagedevice followed by a precious metal catalyst. The engine is operated ata lean air-fuel ratio when a signal from a NOx sensor disposed in theengine exhaust downstream of the NOx storage device indicates an exhaustgas NOx concentration is less than a predetermined threshold. The methodfurther includes determining whether the NOx storage device is full. Inresponse to a determination that the NOx storage device is full, themethod further includes operating the engine at a rich air-fuel ratio.

The method further includes increasing electrical energy to thenonthermal plasma discharge device when a signal from the NOx sensorindicates that exhaust gases contain more than a predeterminedconcentration of NOx and the NOx storage device is not nearly full andincreasing an amount of fuel supplied to the nonthermal plasma dischargedevice when a signal from the NOx sensor indicates that exhaust gasescontain more than the predetermined concentration of NOx and the NOxstorage device is not nearly full, the fuel being supplied by a fuelinjector disposed in the engine exhaust upstream of the nonthermalplasma discharge device and downstream of the injector.

According to another aspect of the invention, a nonthermal plasmadischarge device is operated to convert NO to NO2, by providing thenonthermal plasma discharge device a quantity of fuel and an amount ofelectrical energy. A desired NO to NO2 conversion efficiency isdetermined. The quantity of fuel and the amount of electrical energy tosupply to the nonthermal plasma discharge device are based on thedesired conversion efficiency and minimizing a total effective fuelconsumption by the nonthermal plasma discharge device.

An advantage of the present invention is that the conversion of NO toNO2 is performed without consuming more fuel than necessary. That is,the amount of fuel supplied and the electrical energy are adjusted toprovide the desired conversion rate while minimizing overall fuelconsumption.

A further advantage of the present invention is a method for providingthe desired conversion rate of NO to NO2.

Other advantages, as well as objects and features of the presentinvention, will become apparent from the following detailed descriptionof the preferred embodiments when taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a typical gasoline engine;

FIG. 2 shows a schematic of the exhaust aftertreatment system of theengine shown in FIG. 1;

FIG. 3 is a flowchart showing a method of operating an engine andaftertreatment system according to an aspect of the present invention;and

FIGS. 4 and 5 are flowcharts showing diagnostic procedures to determineoperational activity of the nonthermal plasma discharge device accordingto an aspect of the present invention.

DETAILED DESCRIPTION

A 4-cylinder internal combustion engine 10 is shown, by way of example,in FIG. 1. Engine 10 is supplied air through intake manifold 12 anddischarges exhaust gases through exhaust manifold 14. An intake ductupstream of the intake manifold 12 contains a throttle valve 32 which,when actuated, controls the amount of airflow to engine 10. Sensors 34and 36 installed in intake manifold 12 measure air temperature and massairflow (MAF), respectively. Sensor 24, located in intake manifold 14downstream of throttle valve 32, is a manifold absolute pressure (MAP)sensor. A partially closed throttle valve 32 causes a pressuredepression in intake manifold 12. When a pressure depression exists inintake manifold 12, exhaust gases are caused to flow through exhaust gasrecirculation (EGR) duct 30, which connects exhaust manifold 14 tointake manifold 12. Within EGR duct 30 is EGR valve 18, which isactuated to control EGR flow. Fuel is supplied to engine 10 by fuelinjectors 26. Each cylinder 16 of engine 10 contains a spark plug 26.The crankshaft (not shown) of engine 10 is coupled to a toothed wheel20. Sensor 22, placed proximately to toothed wheel 20, detects engine 10rotation.

Engine 10 is described as a spark-ignition engine. However, the presentinvention applies also to a compression-ignition type engine, whichcould be a homogeneous-charge, compression-ignition or diesel engine

Continuing to refer to FIG. 1, electronic control unit (ECU) 40 isprovided to control engine 10. ECU 40 has a microprocessor 46, called acentral processing unit (CPU), in communication with memory managementunit (MMU) 48. MMU 48 controls the movement of data among the variouscomputer readable storage media and communicates data to and from CPU46. The computer readable storage media preferably include volatile andnonvolatile storage in read-only memory (ROM) 50, random-access memory(RAM) 54, and keep-alive memory (KAM) 52, for example. KAM 52 may beused to store various operating variables while CPU 46 is powered down.The computer-readable storage media may be implemented using any of anumber of known memory devices such as PROMs (programmable read-onlymemory), EPROMs (electrically PROM), EEPROMs (electrically erasablePROM), flash memory, or any other electric, magnetic, optical, orcombination memory devices capable of storing data, some of whichrepresent executable instructions, used by CPU 46 in controlling theengine or vehicle into which the engine is mounted. Thecomputer-readable storage media may also include floppy disks, CD-ROMs,hard disks, and the like. CPU 46 communicates with various sensors andactuators via an input/output (I/O) interface 44. Examples of items thatare actuated under control by CPU 46, through I/O interface 44, are fuelinjection timing, fuel injection rate, fuel injection duration, throttlevalve 32 position, spark plug 26 timing, and EGR valve 18. Various othersensors 42 and specific sensors (engine speed sensor 22, engine coolantsensor 38, manifold absolute pressure sensor 24, air temperature sensor34, and mass airflow sensor 36) communicate input through I/O interface44 and may indicate engine rotational speed, vehicle speed, coolanttemperature, manifold pressure, pedal position, cylinder pressure,throttle valve position, air temperature, exhaust temperature, exhauststoichiometry, exhaust component concentration, and air flow. Some ECU40 architectures do not contain MMU 48. If no MMU 48 is employed, CPU 46manages data and connects directly to ROM 50, RAM 54, and KAM 52. Ofcourse, the present invention could utilize more than one CPU 46 toprovide engine control and ECU 40 may contain multiple ROM 50, RAM 54,and KAM 52 coupled to MMU 48 or CPU 46 depending upon the particularapplication.

The exhaust aftertreatment system 80 coupled to engine 10 is shown inFIG. 2. A nonthermal plasma discharge device (NPDD) 56 is locateddownstream of the exhaust manifold 14. Downstream of NPDD 56 is a leanNOx trap (LNT) 58. Downstream of LNT 58 is a catalyst containingprecious metals on its internal surfaces. NPDD 56 converts NO to NO2,but does so at higher efficiency in the presence of hydrocarbonmaterials. Thus, preferably, an injector 66 is placed upstream of NPDD56 supplying fuel or another hydrocarbon. Alternatively, engine 10 is adirect injection engine in which fuel injectors 26 provide fuel intocylinders 16. In a DI engine, fuel can be supplied to NPDD 56 byinjecting after the combustion has occurred in the cylinder and beforethe exhaust valve closes. Thus, excess fuel is supplied at a time in thecycle in which significant oxidation of the fuel does not occur. Inanother alternative, the combustion process in the cylinder can bespecifically tailored to provide some excess hydrocarbons into theexhaust gases, eg., by fuel stratification. In yet another alternative,a fuel injection pulse from a port fuel injector 26, as shown in FIG. 1,can be caused to occur during valve overlap, i.e., when both intake andexhaust valves are open, allowing fuel to travel through cylinders 16without being combusted.

Continuing to refer to FIG. 2, exhaust aftertreatment system 80comprises exhaust gas sensors. Sensors 64, 68, 70, and 72 are exhaustgas oxygen (EGO) sensors. Alternatively, sensors 64, 68, 70, and 72 areNOx sensors. In a further alternative, there are EGO and NOx sensorslocated at each of 64, 68, 70, and 72. Sensors 64, 68, 70, and 72provide signals to I/O 44 of ECU 40. A signal from ECU 40 controls fuelinjector 66.

It is well known to those skilled in the art that LNT 58 traps primarilyNO2. A typical exhaust gas composition has a NO2/(NO+NO2) ratiosignificantly less than 10%. Thus, if raw exhaust gases were fed to LNT58, only a small fraction of the NOx, i.e., the NO2 portion, would bestored within LNT 58. In the exhaust aftertreatment system 80 shown inFIG. 2, NPDD 56 is placed upstream of LNT 58 to convert NO to NO2.Within NPDD 56, an electrical discharge, in the presence of a smallconcentration of hydrocarbons causes NO to oxidize to NO2. The exhauststream, in which the NO has been converted to NO2, is conducted to LNT58, in which the NO2 is trapped. This continues until LNT 58 no longercan store more NO2, at which point, LNT 58 is purged by causing theair-fuel ratio in the exhaust to become rich. Rich exhaust gases causethe NO2 to desorb from LNT 58. Thus, a purge is initiated by operatingengine 10 at a rich air-fuel ratio. Alternatively, fuel can be added toexhaust gases to cause the overall stoichiometry to be rich.

Precious metal catalyst 60, located downstream of LNT 58, has twofunctions. It oxidizes hydrocarbons, aldehydes, and CO during lean andrich operation. During rich operation, stored oxygen in NO2 serves asthe oxidant, decomposing or reacting with reductants into N2 and O2.During stoichiometric operation, catalyst 60 also reduces NOx.

The efficiency at which NPDD 56 converts NO to NO2 is affected primarilyby two variables: the amount of electrical energy supplied to the NPDD,P_(elec), and the amount of hydrocarbons supplied, m_(f, inj):η_(conv)=function(P _(elec) , m _(f, inj))  (1)Both P_(elec) and m_(f, inj) penalize system fuel economy. The desiredconversion efficiency can be achieved while minimizing fuel economypenalty. The amount of fuel energy consumed in providing the electricalpower to drive the NPDD 56 can be determined from:P _(elec)=(m _(f, eq) *ΔH _(R))/η_(overall)where m_(f, eq) is the equivalent fuel consumed in providing electricalenergy to the NPDD 56, ΔH_(R) is the enthalpy of reaction of fuel, andη_(overall) is the overall efficiency of the engine in converting thefuel's chemical energy into electrical energy and providing that to theNPDD 56. The value of η_(overall) is a function of engine operatingconditions and is so computed. Alternatively, a constant value ofη_(overall) is used if the magnitude of the range in 72 _(overall) overthe engine operating map is inconsequential. The total effective fuelconsumed in the NPDD 56 is:m _(f, tot)=η_(overall) *P _(elec) *ΔH _(R) +m _(f, inj.)  (2)Equation 1 above is solved with the additional constraint thatm_(f, tot), according to equation 2, is minimized. In the abovediscussion, the hydrocarbon supply is defined as fuel. If thehydrocarbon supply is other than fuel, the above equations apply, exceptthat ΔH_(R) is the enthalpy of reaction of the fluid being supplied.

Referring now to FIG. 3, a routine for operating engine 10 starts instep 100. In step 102, the engine is operated at a lean air-fuel ratio.In step 104 it is determined whether [NOx] at the sensor is greater thana [NOx]_(threshold). Preferably, [NOx]_(threshold) is determined as afunction of engine operating condition. If the threshold is exceeded, itis determined whether the NOx trap is likely to be full. If full,control passes to step 108 in which a purge cycle is accomplished bycausing the engine air-fuel ratio to be rich. At the same time, theamount of electrical energy, P_(elec), and the amount of hydrocarbons,m_(f, inj) supplied to NPDD 56 are altered. Preferably, these arecurtailed to save energy during the purge. Alternatively, P_(elec) andm_(f, inj) are operated at a different level than during trapping. TheNOx exiting LNT 58 is conducted into PM catalyst 60, in which NOx isreacted to N2 and O2, step 110. If in step 106 it is determined that LNT58 is not full, one or both of steps 112 and 114 occur: increasing P andincreasing m_(f, inj) to NPDD 56. Both steps 112 and 114 are shown asconsequences of a negative result from step 106. Alternatively, one ofsteps 112 and 114 can be accomplished. Then, in step 104, it isdetermined whether the action taken in step 112 or 114 caused a decreasein [NOx] below the threshold level. If not, the other of steps 112 and114 is caused to occur.

According to another embodiment of the present invention, the loop inFIG. 3, comprising steps 102, 104, 106, 112, and 114, can be used toupdate the constants in the operating model of NPDD 56, according toequation 1. In yet a further embodiment of the present invention,accessing steps 112 and 114 in FIG. 3 indicates that the system is notproviding the expected conversion of NO to NO2 in NPDD 56. As mentionedabove, one alternative is to adjust the model. Another alternative is toaccess a system diagnostic routine when steps 112 and 114 are repeatedlyaccessed with limited improvement in NO conversion efficiency in NPDD56.

A diagnostic routine for NPDD 56 is shown in FIG. 4. Beginning in step198, the diagnostic routine is initiated. The diagnostic routine isbegun only when the LNT has been recently purged. That is, operation ofthe diagnostic routine is undertaken at a time when a high level of NOxat NOx sensor 70 would signify a problem with NPDD 56 converting NO toNO2, not a problem with LNT being unable to trap NO2. If the startingcondition is met, power supply to NPDD 56 is that determined from anengine model of NPDD 56 performance. Based on the present engineoperating condition, the model provides an expectation for electricalpower and fuel to be supplied to NPDD 56. Alternatively, a lookup tablebased on engine operating conditions is used to determine electricalpower and fuel to supply to NPDD 56. Power to NPDD 56 is reduced in step202. In step 204, it is determined whether the NOx concentration atsensor 70 has increased. If there has been an increase in NOxconcentration, it is determined that NPDD 56 is working, step 206. Thediagnostic is ended in step 212. If the result from step 204 isnegative, that is NOx concentration has not changed in response to achange in power to NPDD 56, the model is adjusted in step 214. Next, itis determined in step 208 whether NPDD 56 power is zero. If so, controlpasses to step 210 in which it is registered that NPDD 56 is notworking. If a negative result from step 208, control passes to step 202to further reduce NPDD 56 power. The power is progressively reduceduntil NPDD 56 power is turned off.

The purpose of progressively lowering power to NPDD 56, as shown in theloop of steps 202, 204, and 208, is to ensure that NPDD 210 is truly notworking. In one scenario, if the power to NPDD 56 is higher than needbe, then dropping the power does not result in a measurable differencein NOx concentration at the exit of LNT 58. Thus, to obtain an accuratedetermination of NOx concentration this possible scenario is ruled outby steps 202, 204, and 208.

An alternative to the diagnostic method of FIG. 4 is to instead turn offpower to NPDD 56 in step 202, i.e., turn it off completely rather thanprogressively reduce NPDD 56 power. Steps 208 and 214 are unnecessary inthe alternative. If a negative result is returned from step 204, controlpasses directly to step 210. The difference between FIG. 4 and thealternative to FIG. 4 is that by turning off NPDD 56 completely, the NOto NO2 conversion is completely turned off. During even a short intervalin which NO is not converted to NO2, NO breaks through exhaustaftertreatment system 80 leading to a momentary increase in exhaustemissions. By reducing the power to NPDD 56 according to the diagnosticmethod shown in FIG. 4, NO to NO2 conversion is lessened but notcompletely stopped. The emission impact of the FIG. 4 diagnostic is lessthan the alternative method.

An alternative diagnostic strategy in which fuel supply is reduced isshown in FIG. 5. The steps are analogous to those in FIG. 4, expect thatit refers to fuel supply to NPDD 56.

While several modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize alternative designs and embodiments for practicing theinvention. Thus, the above-described embodiments are intended to beillustrative of the invention, which may be modified within the scope ofthe following claims.

1. A method for operating an internal combustion engine, comprising:providing an exhaust aftertreatment system coupled downstream of theengine, such exhaust aftertreatment system having an injector, anonthermal plasma discharge device located downstream of the injector, aNOx storage device located downstream of the nonthermal plasma dischargedevice, and a NOx sensor located downstream of the NOx storage device;determining a desired NO to NO2 conversion efficiency; providing aquantity of fuel and a quantity of electrical energy to the nonthermalplasma discharge device based on said desired conversion efficiency andminimizing a total effective fuel consumption by the nonthermal plasmadischarge device; and: determining said total effective fuel consumptionas a sum of the quantity of fuel supplied to the nonthermal plasmadischarge device and an equivalent fuel consumption quantity, saidequivalent fuel consumption quantity is based on an amount of electricalpower supplied to the nonthermal plasma discharge device, the overallefficiency of the engine to convert fuel energy into electrical energy,and the energy content of the fuel.
 2. The method of claim 1 whereinsaid overall efficiency of the engine is determined based on engineoperating conditions.
 3. A method to operate a nonthermal plasmadischarge device in converting NO to NO2, the nonthermal plasmadischarge device being a component included in an exhaust aftertreatmentsystem coupled to an internal combustion engine, the method comprising:supplying a quantity of fuel to said nonthermal plasma discharge device;supplying a quantity of electrical energy to the nonthermal plasmadischarge device; basing said fuel quantity and said electrical energyquantity on minimizing a total effective fuel consumption of thenonthermal plasma discharge device wherein said total effective fuelconsumption is based on a sum of said fuel quantity and an effectivefuel consumption to provide said quantity of electrical energy.
 4. Themethod of claim 3 wherein said effective fuel consumption is based onthe overall efficiency of the engine to convert fuel energy intoelectric energy and the energy content of the fuel.
 5. The method ofclaim 3 wherein said exhaust aftertreatment system further comprises aNOx storage device located downstream of the nonthermal plasma dischargedevice.
 6. A method for operating an exhaust aftertreatment systemcoupled to an internal combustion engine, comprising: increasing aquantity of fuel supplied to the exhaust aftertreatment system when asignal from a NOx sensor disposed proximate the exhaust aftertreatmentsystem indicates NOx exceeds a predetermined level of NOx; andincreasing electrical energy to said nonthermal plasma discharge devicebased on a signal from said NOx sensor, wherein said exhaustaftertreatment system comprises a nonthermal plasma discharge devicelocated downstream of said engine, a NOx storage device locateddownstream of said nonthermal plasma discharge device, and a NOx sensorlocated downstream of said NOx storage device and said increases in saidelectrical energy and increases in said quantity of hydrocarbons arecoordinated.
 7. The method of claim 6 wherein said coordination is basedon minimizing a fuel economy penalty.
 8. The method of claim 7 whereinsaid coordination is further based on maintaining a NOx concentration atthe NOx sensor below a predetermined NOx concentration.